In this eNewsletter we continue our discussion of calculating fuel economy for light duty cars and trucks. We present the results of two studies.

The first study is on an employee’s 2010 Prius HEV. The second study is from an OEM’s fleet of 111 light-duty vehicles.

The Prius study took place over 6 months of on-road driving.

Conclusions from Prius Testing are:

• Fuel injector data agrees well with fuel tank after 30 gal of fuel is added
• MAF based results show 5% higher fuel economy than data from fuel tank & injector data
• MAF based data correlates with the instrument panel display
• Therefore, instrument cluster MPG is likely 5% high

The second study is from an OEM’s fleet of 111 LD vehicles. HEM Data calculated the Fuel Economy on a Web Dashboard for the study. Vehicles were located in several states including: CA, GA, IL, LA, MA, MD, MO, NY, OH, OR, and TX. Because of the geographic diversity and length, the study included both warm and cold weather states. The test period was from December 2011 to January 2013.

Summary of Differences between Fuel injector and MAF-based results:

• Average difference = 2.8%
• Std Dev = 5.1%
• Mode = 1.2%
• 60 of the 111 vehicles were within +5%
• 32 of the 111 vehicles were within +3%
• Summary chart % differences by engine model ranged from -5.2 to +7.9%.

Conclusions from Fleet Study

• Using MAF and A/F ratio is a valid alternative to calculating fuel consumption and fuel economy if injector data is not available
• MAF approach generally shows lower fuel consumption and higher fuel economy
• 25 of 111 vehicles showed the opposite result
• Consistent results with hybrid vehicles as the first study shows
• MAF-based calculation error could be 10% without noticing it on a single vehicle
• Some MAF Sensors for this OEM report 0 g/s intermittently producing invalid data
• If injector data is not available, then to gain confidence compare MAF calculations to either:
  • Fuel tank additions of more than 30 gal, or
  • Compare to a larger vehicle population
• Could use a known factor to adjust the results to better match the injector results as NREL did in their study of heavy duty vehicles (SAE paper 2012-01-2049, Measured Laboratory & In- Use Fuel Economy… by M.P. Lammert, et al.)

Study is available at http://fuel.hemdata.com/